What can I do to stop weeds from growing between patio square slabs?

you can try laying down weed mat beneath the slabs and then filling in with sand- or if this is in a sunny spot an organic method is to spray with vinegar!

Make a mixture of white vinegar and water, add salt and dish soap

Spray it with vinegar salt and dawn between slabs weekly or as needed first sight of weed

Boiling water will kill the weeds. Vinegar will do the same. Ove time, you will still need to do that once in a while because weeds are persistent.

Spritz with vinegar as they appear. Just do not spray your lawn or plants. Boiling water will also do the trick.

Unless you grow low growing plants there instead (seeds) Camermile etc. you will have a job for life Use a weed killer.

I agree Sedum or Chamomile or any low growing ground cover plant would be pretty, the Chamomile also smells lovely, weeding is a pain, the sedum etc will be controlled by footfall or can always be scraped now and again.